Managing personal Outlook.com account settings, security, and privacy
Hello,
Do you have any other verification method besides the Unknown email address?
If the email address is the only recovery option available and you don't have access to it, you will have to replace the Security Information. That is, on the Verify Your identity screen, select 'I don't have any of these' and follow the prompts to replace it.
Note -
If you remove all of your previous security information and replace it with something new, your account will be put into a restricted state for 30 days.